The invention relates to a method for the protected transmission of, in particular, relatively long data sequences, the coding of which is represented by a sequence of a predefined number of on and off values. The invention further relates to a mobile data memory, a read-write device for carrying out the method and an identification system with the read-write device and at least one mobile data memory.
Contactless identification systems are based on contactless transmission methods. These methods can be based, e.g., on electromagnetic means, using infrared or ultrasound technology. Such systems are used, for example, to identify persons or moving goods. For this purpose, the necessary data are transmitted from a read-write device via a contactless data transmission link, e.g., an air interface, to a mobile data memory and back again. The contactless identification method also makes it possible to acquire data, e.g., while the mobile data memory moves past a read-write device. To enable the mobile data memories to be used for an indefinite period of time, the integration of energy storage devices, e.g., batteries, is dispensed with. The necessary electric power is picked up externally, i.e., from an electric or magnetic field originating from the read-write device.
To enable communication of a read-write device with such mobile data memories, suitable transmission and coding methods are required, which ensure the power supply of the electronics on the mobile data memory as well as compliance with radio regulations. In addition, only certain frequency bands are typically available for the transmission of data, e.g., the ISM (Industrial, Scientific & Medical) frequency bands for industrial, scientific and medical applications. To ensure a continuous power supply, a carrier frequency modulated with the data to be transmitted is switched off only for a maximum time interval. Within this time period, a previously charged energy storing device in the mobile data memory must be capable of bridging the power supply. Conversely, data is transmitted from the mobile data memory to the read-write device using load modulation. Load modulation can be continuous for a maximum time interval or, as an alternative, carrier frequency-modulated with an auxiliary carrier. Such methods are known, for example, from ISO/IEC Standard 15693, Part 2, “Air Interface and Initialization,” or as time slot methods for operation in an ISM frequency band in accordance with ISO/IEC Standard 14443.
Data transmission between a read-write device and a mobile data memory may be subject to interference, however. In the case of data transmission by way of inductive coupling, for example, this could be electromagnetic interference sources, e.g., motors, solenoid valves, welding robots, etc., which are operated in the immediate vicinity. This can result in faulty data transmission.
To reduce this problem, suitable protection methods are known in the art, e.g., generating and appending a CRC (Cyclic Redundancy Check) word or a parity bit to the end of the data or the data sequence to be transmitted.
Using a parity bit for the protected transmission of data keeps the computational cost extremely low compared with generating a CRC word. Consequently, the error detection probability for a faulty data sequence is also not very high. If during data transmission, two time slots, for example, are faulty within a transmitted sequence, then the value of the parity bit does not change if the values of the time slots are complementary.
The applicant has previously filed a patent application (Application No. 10214188.6) for a solution to this problem. The object stated therein was attained by a method for the protected transmission of data, the coding of which is represented, respectively, by a sequence of a predefined number of on and off values. A count value representing the predefined number is formed by changing the count direction after each on value and incrementing or decrementing the count value with each off value. An error information is generated if a first end value, which is transmitted as a coded sequence of the count value together with the data, differs from a second end value, which, like the count value, is formed from the transmitted sequence. This will be briefly explained again with reference to FIG. 3.
The method described in the aforementioned patent application is advantageous for short data sequences. A fault can be readily detected with the aid of the coded transmission and the partial signature. This method enables a coded transmission of a few bytes of data information.
For longer data sequences, that is to say, with an increasing number of transmitted time slots, the probability of an error in the transmitted data sequence increases. Starting from a length of approximately 20 bytes of coded data to be transmitted, this method is no longer advantageous.
The resulting problem is that the use of the above-described method in identification systems consisting of mobile data memories and read-write devices remains limited to a few bytes of coded data to be transmitted.